1. Field of the Invention
The present invention relates to a semiconductor device having a circuit which includes a thin film transistor, and a method for manufacturing the semiconductor device. For example, the present invention relates to an electronic appliance on which a light-emitting display device including an organic light-emitting element or an electro-optic device typified by a liquid crystal display panel is mounted as a component.
Note that the term “semiconductor device” in this specification refers to a device in general that can operate by utilizing semiconductor characteristics, and an image display device, an electro-optic device, a semiconductor circuit, and an electronic appliance are all included in the semiconductor device.
2. Description of the Related Art
In recent years, a technique for forming thin film transistors (TFTs) each using a semiconductor thin film (a thickness of several nm to several hundred nm) formed over a substrate having an insulating surface has attracted attention. The thin film transistors are widely applied to electronic devices such as ICs and electro-optic devices and are particularly expected to be rapidly developed as switching elements of image display devices.
For a switching element in an image display device, a thin film transistor using an amorphous semiconductor film, a thin film transistor using a polycrystalline semiconductor film, or the like is used.
In the case of a thin film transistor using an amorphous semiconductor film, an amorphous semiconductor film such as a hydrogenated amorphous silicon film is used; therefore, there is limitation on the process temperature and heating at a temperature of 400° C. or higher at which hydrogen is desorbed from the film, intense laser light irradiation by which surface roughness is caused due to hydrogen in the film, and the like are not performed. The hydrogenated amorphous silicon film is an amorphous silicon film whose electric characteristics are improved by bonding of hydrogen to dangling bonds and disappearance of the dangling bonds as a result.
A thin film transistor using a polycrystalline semiconductor film has an advantage that mobility is higher than that of a thin film transistor using an amorphous semiconductor film by two or more digits, and a pixel portion and a peripheral driver circuit of a display device can be formed over one substrate. However, because of crystallization of a semiconductor film, steps of a thin film transistor using a polycrystalline semiconductor film are more complicated than steps of a thin film transistor using an amorphous semiconductor film. Therefore, there occurs a problem in that yield is reduced and cost is increased.
Patent Document 1 (U.S. Pat. No. 5,591,987) discloses a FET (field effect transistor) in which a channel formation region is formed of a semiconductor having a mixture of a crystal structure and an amorphous structure.
Further, as a switching element in an image display device, a thin film transistor using a microcrystalline semiconductor film is used (Patent Document 2: Japanese Published Patent Application No. H4-242724 and Patent Document 3: Japanese Published Patent Application No. 2005-49832).